ACROSS
 1. <play accompaniment> VAMP = AMP after V
 4. AN + TITHES IS = ANTITHESIS <opposition>
 9. <videogame> PLATFORMER = P_L AT FORMER
10. <cereals> RYES = "RISE"
11. <picture> VIGNETTE = GIVEN* T_T_E
12. R inside A SHAM = ASHRAM <retreat>
13. (.n)CE DI(r.) = CEDI <Ghanaian currency>
14. S T (LAG after A) MITE = STALAGMITE <projection>
16. <visitor> HOUSEGUEST = (.t)HOU SEGUES T(e.)
18. <food> NOSH = S_ inside NOH
20. E FT outside FOR = EFFORT <work>
22. <displays> TABLEAUX = TAB L_ EA UX
24. <neck> SNOG = S NOG
25. EG LINES outside A_ N_ T_ = EGLANTINES <climbers?>
26. <mint> PENNYROYAL = PENNY ROYAL
27. "MEAN" = MIEN <appearance>

DOWN
 2. (TEAM ILL)* = ALL-TIME <without match? {unsurpassed}>
 3. PROUSTIAN* = PUT ON AIRS <seemed pretentious>
 4. FLOAT* = ALOFT <upwards>
 5. TEMPE[-h] + RAMEN TALLY = TEMPERAMENTALLY <vis-a-vis vibes>
 6. <rent> TORE APART = TO REAP + ART
 7. [-d]EARTH = EARTH <soil>
 8. I + NEXT outside A_ C_ = INEXACT <ballpark>
14. (HE GETS)* inside SIR = SIGHTSEER <tourist>
15. MAN ←M SIREN← = MANNERISM <behaviour>
17. <wrongdoing> OFFENCE = OF FENCE
19. (THEY USE)* = SHUT-EYE <sleep>
21. <wind instrument> ORGAN = _O RANG*
23. <conventional> BANAL = N inside BAAL